Thermal analysis of goethite - Relevance to Australian indigenous art

Differential scanning calorimetry shows two endotherms at 75 and 225degreesC for synthetic goethite. The latter endotherm is strongly asymmetric on the low temperature side. The endotherms were attributed to the loss of water and the dehydroxylation of the goethite. The temperature of the endotherms and the enthalpy of the phase change were found to be linear functions of the percentage of aluminium substitution into the goethite. High-resolution thermogravimetric analysis of goethite showed three mass loss steps, occurring at similar to 175, 196 and 263degreesC. The temperatures of these mass loss steps and the percentage of mass loss were also linearly related to the degree of Al substitution. The use of infrared emission spectroscopy confirmed the temperature of dehydroxylation. The observation of the low temperature dehydroxylation of goethite and its relation to ancient aboriginal cave art is discussed.